1. Technical Field
This invention relates generally to the field of neural network models and artificial intelligence. More specifically, this invention relates to a model of the computational function of a single artificial neuron.
2. Description of the Related Art
There has long been an interest in developing artificial systems that exhibit the intelligence of humans or other animals. One means of approaching this challenge has been to create computational systems that incorporate known elements of biological nervous systems. This approach has led to the creation of artificial neural networks. An artificial neural network consists of multiple inter-connected “neurons,” in which the output of each neuron depends on the weighted sum of its inputs. The network as a whole receives external inputs and generates outputs. The goal of the system is to learn to generate appropriate outputs to each input pattern by adjusting the connection weights between neurons. The adjustment of connection weights may occur under the influence of some sort of feedback about the appropriateness of the output.
Neural networks have had some success in generating useful intelligence. But despite the fact that they have been around since the 1950's, significant efforts have failed to result in artificial intelligence that compares to that generated by biological nervous systems. Likewise, the neurons of artificial networks bear only a limited resemblance to biological neurons.
Biological nervous systems have the ability to learn for themselves (i.e. self-organize). A primary goal of research on neural networks has been to create artificial networks that can do the same. Ideally, a network would start with virtually no information about the world but would become knowledgeable over time on its own. Whether or not this could ever be achieved, a practical goal is to develop networks that are as general as possible in what they are able to learn, while having as little information as possible built in from the start.
Neural networks have typically been designed using substantial knowledge of how to solve a specific task. This is most obvious in the case of networks trained under the influence of an externally generated supervisory signal, which essentially tells the network what it is doing wrong. Thus the network is fed expert information. By contrast, biological nervous systems must learn for themselves through trial and error. Likewise, some artificial networks perform unsupervised learning, which by definition does not depend on an external supervisory signal. However, even these networks have generally been designed so that the network learns to perform a particular input-output transformation. Thus the designer builds the network with knowledge of how outputs should depend on inputs. Since the appropriate input-output transformation will be different for different tasks or environments, a network designed for one environment may not be able to adapt to a new environment. By contrast, biology discovers for itself how outputs should depend on inputs, and biological nervous systems can be remarkably adaptable to new environments. The present ideas are based on a computational model of biological neurons.
The goal of a network is to select appropriate outputs, a process that could be referred to as “decision making.” The problem in making decisions is uncertainty about the current and future states of the world. The uncertainty in a prediction is inversely related to the amount of information upon which the prediction is based, and therefore minimizing uncertainty is the same as maximizing information. If the system could accurately predict the state of the world, then the problem would be solved, and the system would merely select the output that it knows is the most advantageous (e.g. the output that maximizes its expected future reward). The process of minimizing uncertainty is thus considered to be formally identical to the process of decision-making, since decisions are rendered trivial in the absence of uncertainty.
Any reference made to “information” herein refers to information that the network or a neuron holds within its biophysical structure. Even a single molecule, such as an ion channel in a neuronal membrane, contains information. By definition, information predicts something. Thus the prediction made by a neuron is entirely conditional on information possessed by the neuron. In the present work, the relevant information is not that which an observer of the network may have about the network or the world.
The present invention can be viewed as an application of fundamental principles of reinforcement learning to the computational function of a single neuron. Reinforcement learning is concerned with how a biological or artificial agent can learn on its own to achieve its goals. Since all of an agent's outputs should be selected in order to promote its future goals, an agent learns to predict “the sum of future rewards.” Although the notion of future rewards is quite abstract and intangible, the agent predicts future reward by predicting concrete stimuli and events that are themselves predictive of future reward. Thus the agent tries to predict the current state of the sensory world because the sensory world is informative about future reward. For example, the sight of food is predictive of the ingestion of food, and thus an agent would like to be able to predict the sight of food.
To learn to make such predictions, the agent relies on prediction errors. A prediction error is the difference between what is expected and what is actually observed. The artificial neuron described here generates a type of prediction error, and uses it to learn to predict an aspect of the world that is itself potentially predictive of future reward. Whereas most past work on reinforcement learning has implemented principles of reinforcement learning at the level of networks and systems, the present work applies these principles to the function of a single neuron, and by extension, to a network of neurons.
Artificial neural networks are composed of multiple interconnected neurons. The output of each neuron is determined by the weighted sum of its inputs. Each input serves as an information source, and in principle, a neuron could have a virtually infinite number of inputs. But a neuron's output depends only on those inputs that have sufficiently high weights. Creating an intelligent network is therefore largely a matter of selecting the appropriate weights. Despite over 50 years of work on this problem, there has not been a generally accepted and universally applicable set of guidelines that specifies the weight that should be given to each of a neuron's inputs. Herein, new principles are described for choosing the weights of a neuron's inputs. Some inputs should be given high weights if they are unpredictable from the neuron's perspective, whereas others should be given high weights if they are effective predictors.
Although there are multiple means by which these new principles might be implemented, perhaps the most attractive embodiment would utilize plasticity algorithms. Real neurons communicate with one another through synapses in which information is traditionally thought to flow in one direction from the pre-synaptic to the post-synaptic neuron. The strength of the connection between two neurons is described by a weight, and many artificial neural networks employ plasticity algorithms that modify the weight according to coincident activity in the pre-synaptic input neuron and the post-synaptic output neuron. A commonly used class of plasticity rules is known as Hebbian. According to a Hebbian rule, a connection weight increases if activity in a pre-synaptic neuron is positively correlated with the activity of a post-synaptic neuron. Thus, if a pre-synaptic neuron excites a post-synaptic neuron at approximately the same time that the post-synaptic neuron is excited, then that connection weight increases. If the pre-synaptic neuron is excited at the same time that the post-synaptic neuron is inhibited, then the connection weight may decline.
A less often used form of plasticity in artificial neural networks is known as “anti-Hebbian.” A Hebbian rule strengthens excitatory or inhibitory inputs that are paired with excitation or inhibition, respectively, and it therefore involves positive feedback. An anti-Hebbian rule does just the opposite and results in negative feedback. Thus an inhibitory input that is active at the same time that the post-synaptic neuron is excited would have its connection weight increased. Anti-Hebbian rules are known to be effective in decorrelating neural activity so as to promote efficient coding, and similarly, in learning to make predictions.
Although both Hebbian and anti-Hebbian rules have been explored in the literature for at least several decades now, the application of these plasticity rules to neural networks has not led to a general solution to the problem of creating artificial intelligence. Although each of these plasticity rules has been shown to have certain virtues, it has not been clear which type of plasticity rule should be applied to a particular neuron or to any of its individual inputs. Herein it is proposed that a particular combination of Hebbian and anti-Hebbian plasticity rules within a single neuron will be useful in creating artificial networks that learn to generate intelligent outputs.
In focusing exclusively on synaptic connectivity between neurons, most work on artificial neural networks has ignored an important aspect of biological neurons. The output of a biological neuron is not determined solely by its synaptic inputs. An approximately equal contribution is made by non-synaptic ion channels. The conductance of many of these ion channels is regulated by membrane voltage. An example would be a voltage-gated potassium channel that opens and inhibits the neuron once the membrane becomes depolarized. There are many different types of voltage-gated potassium channels that can be produced by each neuron, but somehow a particular neuron selectively expresses some types of potassium channels but not others. The rules that govern this selection process in biological neurons are not known, and the neurons of most artificial networks do not possess any analogue of these non-synaptic ion channels. The present invention identifies a key role for non-synaptic ion channels, and utilizes anti-Hebbian plasticity to select amongst different types of non-synaptic ion channel.